The present invention generally relates to the art of intermingling two or more dissimilar portions of material with the aim of attaining a desired level of uniformity in the resulting mixture or blend, and specifically to an apparatus for continuous distribution of at least one additive in a stream of particles of a master or base material.
Numerous and different types of mixing or blending devices are known for continuous intermingling of a particulate solids material as the main portion or base with a minor portion of an additive. In general, such prior art devices are intended for specific uses and specific types of materials. As long as the components of an intended mixture or blend are capable of forming relatively stable mixtures or blends, i. e. when the specific weights of the constituents, their bulk densities, particle sizes and other properties differ within reasonable limits, homogeneous distributions of an additive in a particulate solids base can be achieved in continuous operation by relatively simple devices.
When considering, as an example, a system consisting of a predominant portion, e.g. 90% by weight or more, of smooth and relatively large spheres, e.g. having diameters in the range of from 5 to 20 mm, and of a minor portion, e.g. 10% by weight or less, of a finely divided pulverulent material, e.g. having a particle size of less than 100 micrometers, it will be easily understood that such a system will not be capable to form blends of some stability even if the specific weights of the constitutents of the system are identical. In the absence of mutual surface adhesion, or without an auxiliary phase, such a system can be said to be "immiscible" in the sense that any homogeneous distribution of the minor portion in a given space contaning the other portion will dehomogenize spontaneously, e.g. by the impact of gravity, so as to result in different space portions with undue high concentrations of the one or the other portion.
Conventional devices of the paddle mixer type are not capable of improving interdispersion of the constituents of such systems, neither by increasing the mixing intensity (e.g. increased rpm of a rotating mixing element) nor by increasing the mixing period. In practice, a somewhat uniform mutual distribution of the constituents can be achieved by distributing both components on a common carrier surface so that the constituent with the larger particle size is provided substantially in the form of a "two-dimensional" layer (average layer thickness = average particle diameter), e.g. by means of an endless conveyor belt, and subsequent discharge of the "surface-blend" into a processing apparatus. Screw mixers for continuous operation may also be used according to this surface-distribution mechanism. Application of this mechanism for practical mixing purposes is limited and prior art devices operating in this fashion are quite large, or require a complicated and costly structure.
A commercially important solids/solids distribution problem of the type mentioned above is continuous intermingling of a minor portion of an additive consisting of comparatively small particles, such as pigment powders or the like, in a granulate, e.g. a particulate thermoplastics. Bulk-dispersion of the additive in a thermoplastic material, e.g. pigment addition to a thermoplastics mass in connection with production and subsequent granulation of the mixture, is a conventional method of avoiding this problem but this method conflicts with the requirements of many processors for a reduced diversification of storage items, an economic operation and an easy change of compounding formulae.
Various processing devices for continuous operation and with the aim to solve the above problem are known and include a stationary mixing zone or mixing chamber that includes a rotary mixing element.
Swiss Pat. No. 554,229, for example, discloses metered addition of a finely divided pigment to a relatively coarse plastics granulate by means of a metering screw for pre-blending the pigment and the granulate and introducing the pre-blended material into a blending zone provided within a stream of the granulate for distribution therein by means of a post-blending element that rotates around a horizontal axis. The inlet for the granulate stream is provided above the rotary blending element. As a consequence, the blending zone is subject to the hydrostatic head or pressure of a column of superimposed granular solids. This is disadvantageous for operation as it limits the blending homogeneity that can be achieved with a given drive power, and because it requires a relatively costly apparatus.
A so-called automated coloring device for plastics processing is disclosed in "Kunststoffe-Plastics" (Carl Hanser Verlag, Munich), Volume 21 (1974), pages 29-31, and includes a blending chamber for lateral introduction of granulate as well as a rotary blending element that rotates around a vertical axis. While the disadvantages of hydrostatic pressure inpact upon the blending zone is avoided, the effect of the blending element varies throughout the cross-section of the processed stream.